Christopher Hough

Induction of Mossy Fiber→CA3 Long-Term Potentiation Requires Translocation of Synaptically Released Zn2+

The mammalian CNS contains an abundance of chelatable Zn2+ sequestered in the vesicles of glutamatergic terminals. These vesicles are particularly numerous in hippocampal mossy fiber synapses of the hilar and CA3 regions. Our recent observation of frequency-dependent Zn2+ release from mossy fiber synaptic terminals and subsequent entry into postsynaptic neurons has prompted us to investigate the role of synaptically released Zn2+ in the induction of long-term potentiation (LTP) in field CA3 of the hippocampus. The rapid removal of synaptically released Zn2+ with the membrane-impermeable Zn2+ chelator CaEDTA (10 mm) blocked induction of NMDA receptor-independent mossy fiber LTP by high-frequency electrical stimulation (HFS) in rat hippocampal slices. Mimicking Zn2+ release by bath application of Zn2+ (50–100 μm) without HFS induced a long-lasting potentiation of synaptic transmission that lasted more than 3 hr. Moreover, our experiments indicate the effects of Zn2+ were not attributable to its interaction with extracellular membrane proteins but required its entry into presynaptic or postsynaptic neurons. Co-released glutamate is also essential for induction of LTP under physiological conditions, in part because it allows Zn2+ entry into postsynaptic neurons. These results indicate that synaptically released Zn2+, acting as a second messenger, is necessary for the induction of LTP at mossy fiber→CA3 synapses of hippocampus.

Stress impairs α1A adrenoceptor-mediated noradrenergic facilitation of GABAergic transmission in the basolateral amygdala

Intense or chronic stress can produce pathophysiological alterations in the systems involved in the stress response. The amygdala is a key component of the brains neuronal network that processes and assigns emotional value to lifes experiences, consolidates the memory of emotionally significant events, and organizes the behavioral response to these events. Clinical evidence indicates that certain stress-related affective disorders are associated with changes in the amygdalas excitability, implicating a possible dysfunction of the GABAergic system. An important modulator of the GABAergic synaptic transmission, and one that is also central to the stress response is norepinephrine (NE). In the present study, we examined the hypothesis that stress impairs the noradrenergic modulation of GABAergic transmission in the basolateral amygdala (BLA). In control rats, NE (10 μM) facilitated spontaneous, evoked, and miniature IPSCs in the presence of β and α2 adrenoceptor antagonists. The effects of NE were not blocked by α1D and α1B adrenoceptor antagonists, and were mimicked by the α1A agonist, A61603 (1 μM). In restrain/tail-shock stressed rats, NE or A61603 had no significant effects on GABAergic transmission. Thus, in the BLA, NE acting via presynaptic α1A adrenoceptors facilitates GABAergic inhibition, and this effect is severely impaired by stress. This is the first direct evidence of stress-induced impairment in the modulation of GABAergic synaptic transmission. The present findings provide an insight into possible mechanisms underlying the antiepileptogenic effects of NE in temporal lobe epilepsy, the hyperexcitability and hyper-responsiveness of the amygdala in certain stress-related affective disorders, and the stress-induced exacerbation of seizure activity in epileptic patients.

Characterization of extracellular accumulation of Zn2+ during ischemia and reperfusion of hippocampus slices in rat.

The mammalian CNS contains an abundance of chelatable zinc that is sequestered in the vesicles of glutamatergic presynaptic terminals and co-released with glutamate. Considerable Zn(2+) is also released during cerebral ischemia and reperfusion (I/R) although the mechanism of this release has not been elucidated. We report here the real time observation of increase of the concentration of extracellular Zn(2+) ([Zn(2+)](o)), accompanied by a rapid increase of intracellular free Zn(2+)concentration, in the areas of dentate gyrus (DG), CA1 and CA3 in acute rat hippocampus slices during ischemia simulated by deprivation of oxygen and glucose (OGD) followed by reperfusion with normal artificial cerebrospinal fluid. A brief period of OGD caused a sustained increase of [Zn(2+)](o). Subsequent reperfusion with oxygenated medium containing glucose resulted in a further increase of [Zn(2+)](o). Longer periods of OGD caused greater increases of [Zn(2+)](o,) and subsequent reperfusion caused still further increases of [Zn(2+)](o,) regardless of OGD duration. The Zn(2+) chelator CaEDTA (10 mM) significantly reduced the increase of [Zn(2+)] induced by OGD and reperfusion. Significant regional differences of [Zn(2+)](o) over the areas of the DG, CA1 and CA3 were not observed during I/R. Neither sodium channel blockade by tetrodotoxin (2 microM), perfusion with nominally calcium-free medium nor anatomical disassociation of the DG, CA1 and CA3 regions from one another by lesioning affected the increase of [Zn(2+)](o). The non-specific nitric oxide synthase (NOS) inhibitor, Nomega-nitro-l-arginine methyl ester (1 mM), however, blocked the increase of [Zn(2+)](o) during ischemia and reperfusion. The data indicate the important role of NO in causing the release of Zn(2+) during I/R and suggest that NOS inhibitors may be used to reduce Zn(2+)-induced neuronal injury.

Elevated basal and thapsigargin-stimulated intracellular calcium of platelets and lymphocytes from bipolar affective disorder patients measured by a fluorometric microassay

BACKGROUND A number of investigators have reported finding elevated basal and stimulated intracellular calcium levels in the platelets or lymphocytes of bipolar disorder patients. METHODS Intracellular calcium was measured by a micro fura-2 fluorometric method in the platelets and lymphocytes of 30 affective disorder patients and 14 control subjects. RESULTS We observed significantly elevated basal calcium concentrations in bipolar patient platelets and lymphocytes compared to control subjects. Bipolar patient platelet calcium responses to thrombin, serotonin, and thapsigargin were also significantly greater than control subjects. The peak calcium levels of lymphocytes of bipolar patients were greater than control subjects only when stimulated by thapsigargin. There were significant differences between bipolar and unipolar patients in basal and thapsigargin-stimulated calcium measures but not between bipolar I and bipolar II patients. Unmedicated versus medicated calcium measures were not significantly different. We also found little correlation between calcium measures and the severity of mood rating. CONCLUSIONS Using this method, we were able to confirm and extend the work of others, indicating altered intracellular calcium homeostasis in the blood cells of bipolar disorder patients. In addition, our data suggest that storage operated calcium channels may be the source of the elevated intracellular calcium in platelets and lymphocytes of bipolar patients.

Decreased prefrontal CaMKII α mRNA in bipolar illness

Ca2+/calmodulin-dependent protein kinase II (CaMKII) plays critical roles in neurotransmission, synaptic plasticity, learning and memory. The aim of this study was to examine, by in situ hybridization, prefrontal cortical expression of CaMKII alpha mRNA in postmortem brains of unipolar, bipolar, schizophrenic, and control subjects. Compared to controls, bipolar patients had significantly lower levels of CaMKII alpha mRNA in laminae I-VI of Brodmanns area 9 and laminae I-III and VI of area 46. Unipolar patients also exhibited significantly lower levels of CaMKII alpha mRNA in laminae I-IV of area 9 than did controls. The significant decrease in CaMKII alpha mRNA in bipolar patients could be associated with some of the affective and cognitive alterations that have been linked to prefrontal cortical dysfunction in bipolar disorder, although this requires further direct examination.

Expression and agonist-induced down-regulation of mRNAs of m2- and m3-muscarinic acetylcholine receptors in cultured cerebellar granule cells.

Abstract: The regulation and expression of muscarinic acetylcholine receptor (mAChR) mRNA was studied in cultured cerebellar granule cells using Northern blot hybridization. mRNA species for m2‐ and m3‐mAChRs but not ml‐ and m4‐mAChRs were detected in these cells. The expression of mRNAs of both m2‐ and m3‐mAChRs reached a maximum on the tenth day in culture but their expression patterns differed. Treatment of cerebellar granule cells after 8 days in culture with 100 μM carbachol led to differential down‐regulation of the mRNA species of both mAChR subtypes present. Muscarinic receptor antagonists, atropine (1 μM) and pirenzepine (10 μM), prevented carbachol‐induced m3‐mAChR mRNA down‐regulation observed at 8 h. However, exposure to either atropine or pirenzepine alone for 8 h led to a significant up‐regulation of m3‐mAChR mRNA. Thus, the mRNA species for both m2‐ and m3‐mAChR subtypes are differentially expressed in culture and down‐regulated by agonist stimulation. The loss of these mRNA species may play a role in the down‐regulation of mAChR binding sites that occurs after desensitization.

p11 IS UP-REGULATED IN THE FOREBRAIN OF STRESSED RATS BY GLUCOCORTICOID ACTING VIA TWO SPECIFIC GLUCOCORTICOID RESPONSE ELEMENTS IN THE p11 PROMOTER

Posttraumatic stress disorder (PTSD) is one of the most common psychiatric disorders. Despite the extensive study of the neurobiological correlates of this disorder, the underlying mechanisms of PTSD are still poorly understood. Recently, a study demonstrated that dexamethasone (Dex), a synthetic glucocorticoid, can up-regulate p11, known as S100A10-protein which is down-regulated in patients with depression, (Yao et al., 1999; Huang et al., 2003) a common comorbid disorder in PTSD. These observations led to our hypothesis that traumatic stress may alter expression of p11 mediated through a glucocorticoid receptor. Here, we demonstrate that inescapable tail shock increased both prefrontal cortical p11 mRNA levels and plasma corticosterone levels in rats. We also found that Dex up-regulated p11 expression in SH-SY5Y cells through glucocorticoid response elements (GREs) within the p11 promoter. This response was attenuated by either RU486, a glucocorticoid receptor (GR) antagonist or mutating two of three glucocorticoid response elements (GRE2 and GRE3) in the p11 promoter. Finally, we showed that p11 mRNA levels were increased in postmortem prefrontal cortical tissue (area 46) of patients with PTSD. The data obtained from our work in a rat model of inescapable tail shock, a p11-transfected cell line and postmortem brain tissue from PTSD patients outline a possible mechanism by which p11 is regulated by glucocorticoids elevated by traumatic stress.

Do we need zinc to think

Chelatable Zn2+, which is found in the synaptic vesicles of certain glutamatergic neurons in several regions of the forebrain, is released during neuronal activity. Zn2+ exhibits numerous effects on ligand-gated and voltage-dependent ion channels, and released Zn2+ is therefore likely able to modulate synaptic transmission. The physiologically relevant actions of Zn2+, however, have remained unclear. Recent research exploiting improved Zn2+-sensitive optical probes has suggested some intriguing effects for synaptically released Zn2+, including heterosynaptic regulation of N-methyl-D-aspartate (NMDA) receptor function, and a novel role as a trans-synaptic second messenger that may enter postsynaptic neurons to modulate various signal transduction pathways.

Neurobiology of Bipolar Illness: Implications for Future Study and Therapeutics

Many findings implicating prefrontal cortical and limbic areas of the brain and endocrine systems in the neuropathology and pathophysiology of bipolar illness have greatly increased our understanding of the neurobiology of the illness. New imaging techniques such as PET, MRI, SPECT, and MRS have detailed more evidence of specific regional alterations in the brains of bipolar patients than was thought possible just 20 years ago. These methods are beginning to be used to help predict response to treatment. Examining the mechanisms of action of mood stabilizers (such as lithium, carbamazepine, and valproate) has provided clues to potential underlying neurobiological abnormalities in the illness. Recent studies of postmortem brain tissue have begun to confirm prefrontal cortical and limbic neurochemical and microstructural alterations in patients with bipolar illness compared with controls. It is postulated that it is the balance between primary pathological versus secondary adaptive alterations in gene expression in the illness and their enhancement or dampening by pharmacotherapy, that may determine the episodic course of mood fluctuations and remissions. Further examination of the pathophysiology and neurobiology of bipolar illness should lead to both more effective treatments and, potentially, secondary and even primary episode prevention.

Regulation of β-Adrenergic Receptor mRNA in Rat C6 Glioma Cells Is Sensitive to the State of Microtubule Assembly

Abstract: Microtubule disrupter, colchicine, and microtubule stabilizer, taxol, were used to determine whether microtubules play a role in β‐adrenergic receptor mRNA homeostasis and agonist‐induced down‐regulation in C6 glioma cells. Colchicine treatment had significant, differential, time‐dependent effects on constitutive β1‐ and β2‐adrenergic receptor mRNA levels. These effects stemmed from the action of colchicine on microtubules, because β‐lumicolchicine, an inactive isomer, had no effect, and nocodazole, a structurally unrelated microtubule disrupter, had similar effects. Colchicine treatment had little effect on the total number of β‐adrenergic receptor binding sites as measured by (−)‐[125I]iodopindolol binding, but did alter the relative proportion of β1‐ and β2‐adrenergic receptor subtypes. Colchicine also had no effect on basal cyclic AMP levels. In contrast to colchicine, taxol treatment had little long‐term effect on either β1‐ or β2‐adrenergic receptor mRNA levels. Taxol antagonized the effects of colchicine on total binding and mRNA levels. Taxol treatment increased basal cyclic AMP levels fourfold and potentiated (−)‐isoproterenol‐induced cyclic AMP production. Colchicine pretreatment completely inhibited (−)‐isoproterenol‐induced down‐regulation of β1‐adrenergic receptor mRNA, but not that of β2‐adrenergic receptor mRNA. Taxol pretreatment had little effect on isoproterenol‐induced β‐adrenergic receptor mRNA down‐regulation. Colchicine pretreatment also attenuated isoproterenol‐induced receptor down‐regulation and inhibited agonist‐stimulated cyclic AMP production. These effects of colchicine were antagonized by taxol. Whereas the effects of taxol and colchicine on isoproterenol‐induced down‐regulation of β‐adrenergic receptor mRNA are consistent with their effects on cyclic AMP production, those of colchicine in the absence of stimulation must involve other mechanisms. The data demonstrate that the state of microtubule assembly can affect cyclic AMP levels, β1‐ and β2‐adrenergic receptor mRNA, and binding site levels in C6 glioma cells.